1. Field of the Invention
The present invention relates to a motor vehicle steering system by which a steerable vehicle wheel is steered according to operation of an operation member.
2. Description of Related Art
An example of a motor vehicle steering system is an electric power steering apparatus in which steering assist force generated by an electric motor is transmitted to a steering mechanism. An electric power steering apparatus is equipped with a control device which drives an electric motor according to operation torque which is applied to a steering wheel by a driver. Due to this structure, appropriate steering assist force according to the operation torque can be generated.
Another example of a motor vehicle steering system is a steer-by-wire system (refer to EP1097855 A2). In a steer-by-wire system, there is no mechanical connection between a steering wheel and a steering mechanism. In this system, while operation angle of the steering wheel is detected by a sensor, driving force of a steering actuator, which is controlled in accordance with the output of the sensor, is transmitted to the steering mechanism. It is possible to freely set a ratio of a steering angle (steering angle ratio) of the steerable vehicle wheel to an operation angle of the steering wheel. Moreover, control of the steering angle by the control of the steering actuator can be performed independently from operation of the steering wheel. Thus, it becomes possible to perform stabilization control of a vehicle behavior by the steering control.
The steer-by-wire system is not the only a motor vehicle steering system which can change the steering angle ratio. For example, in a variable gear ratio steering system in which a variable transmission ratio unit, which can change rotation transmission ratio, is intervened, variable control of the steering angle ratio is also possible (refer to Japanese Unexamined Patent Publication No. 2006-2805).
In a stabilization control of a vehicle behavior adopted in steer-by-wire system or the like, target yaw rate is calculated based on the vehicle speed and operation angle, and steering angle of the steerable vehicle wheels is feedback-controlled so that difference between detected actual yaw rate and the target yaw rate becomes zero.
In a condition where a vehicle travels straight on a rutted road which is uneven surface road where wheel ruts or wheel track furrows (depressions left after wheels of a vehicle passed) are formed, the conventional stabilization control of a vehicle behavior cannot sufficiently work. That is, on a rutted road, a motor vehicle is prone to wobble and steering is easy to be taken or entrapped. Therefore, a driver must swiftly and dynamically control the steering wheel to settle down the vehicle attitude.
To realize vehicle attitude control, which is equivalent to the steering control by a driver, by use of a conventional stabilization control of a vehicle behavior, feedback gain must be set large to raise the responsiveness enough. This is so because yaw rate generated in a vehicle traveling straight is very small.
However, if the feedback gain is set large, when driving on an ordinary road which is a level road without ruts, a large control amount is generated by minute change of a yaw rate signal caused by swell of the road or vibration of the vehicle. Therefore, this may more likely to increase wobble of the vehicle.